Sampling and sampling distributions

Sampling and Sampling Distributions By Shakeel Nouman M.Phil Statistics Govt. College University Lahore, Statistical Officer

Shakeel NoumanM.Phil Statistics

Sampling and Sampling Distributions

Using Statistics Sample Statistics as Estimators of Population

Parameters Sampling Distributions Estimators and Their Properties Degrees of Freedom Using the Computer Summary and Review of Terms

Sampling and Sampling Distributions5


• Statistical Inference: Predict and forecast values of

population parameters... Test hypotheses about values

of population parameters...Make decisions...

On basis of sample statistics derived from limited and

incomplete sample information

Make generalizations about the characteristics of

a population...

Make generalizations about the characteristics of

a population...

On the basis of observations of a

sample, a part of a population

On the basis of observations of a

sample, a part of a population

5-1 Statistics is a Science of Inference


Democrats Republicans

People who have phones and/or cars and/or are

Digest readers.

BiasedSample

Population

Democrats Republicans

Unbiased Sample

Population

Unbiased, representative sample drawn at random

from the entire population.

Biased, unrepresentative sample drawn from

people who have cars and/or telephones and/or

read the Digest.

The Literary Digest Poll (1936)


• An estimator of a population parameter is a sample statistic used to estimate or predict the population parameter.

• An estimate of a parameter is a particular numerical value of a sample statistic obtained through sampling.

• A point estimate is a single value used as an estimate of a population parameter.

A population parameter is a numerical measure of a summary characteristic

of a population.

5-2 Sample Statistics as Estimators of Population Parameters

A sample statistic is a numerical measure of a summary characteristic

of a sample.


• The sample mean, , is the most common estimator of the population mean,

• The sample variance, s2, is the most common estimator of the population variance, 2.

• The sample standard deviation, s, is the most common estimator of the population standard deviation, .

• The sample proportion, , is the most common estimator of the population proportion, p.

• The sample mean, , is the most common estimator of the population mean,

• The sample variance, s2, is the most common estimator of the population variance, 2.

• The sample standard deviation, s, is the most common estimator of the population standard deviation, .

• The sample proportion, , is the most common estimator of the population proportion, p.

Estimators

X

p̂


• The population proportion is equal to the number of elements in the population belonging to the category of interest, divided by the total number of elements in the population:

• The sample proportion is the number of elements in the sample belonging to the category of interest, divided by

the sample size:

pXN

pxn

Population and Sample Proportions


XX

XXX

XX

XXX

XXX

XXX

XX

Population mean ()

Sample points

Frequency distribution of the population

Sample mean ( )X

A Population Distribution, a Sample from a Population, and the Population and Sample

Means


• The sampling distribution of a statistic is the probability distribution of all possible values the statistic may assume, when computed from random samples of the same size, drawn from a specified population.

• The sampling distribution of X is the probability distribution of all possible values the random variable may assume when a sample of size n is taken from a specified population.

X

5-3 Sampling Distributions


Uniform population of integers from 1 to 8:

X P(X) XP(X) (X-x) (X-x)2 P(X)(X-x)2

1 0.125 0.125 -3.5 12.25 1.531252 0.125 0.250 -2.5 6.25 0.781253 0.125 0.375 -1.5 2.25 0.281254 0.125 0.500 -0.5 0.25 0.031255 0.125 0.625 0.5 0.25 0.031256 0.125 0.750 1.5 2.25 0.281257 0.125 0.875 2.5 6.25 0.781258 0.125 1.000 3.5 12.25 1.53125

1.000 4.500 5.25000

87654321

0.2

0.1

0.0

X

P(X

)

Uniform Dis tributio n (1,8)

E(X) = = 4.5V(X) = 2 = 5.25

SD(X) = = 2.2913

Sampling Distributions (Continued)


• There are 8*8 = 64 different but equally-likely samples of size 2 that can be drawn (with replacement) from a uniform population of the integers from 1 to 8:

Samples of Size 2 from Uniform (1,8)1 2 3 4 5 6 7 8

1 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,82 2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,83 3,1 3,2 3,3 3,4 3,5 3,6 3,7 3,84 4,1 4,2 4,3 4,4 4,5 4,6 4,7 4,85 5,1 5,2 5,3 5,4 5,5 5,6 5,7 5,86 6,1 6,2 6,3 6,4 6,5 6,6 6,7 6,87 7,1 7,2 7,3 7,4 7,5 7,6 7,7 7,88 8,1 8,2 8,3 8,4 8,5 8,6 8,7 8,8

Each of these samples has a sample mean. For example, the mean of the sample (1,4) is 2.5, and the mean of

the sample (8,4) is 6.

Sample Means from Uniform (1,8), n = 21 2 3 4 5 6 7 8

1 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.52 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.03 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.54 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.05 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.56 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.07 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.58 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0



Sampling Distribution of the Mean

The probability distribution of the sample mean is called the sampling distribution of the the sample mean.

8 .07.57 .06.56 .05.55 .04 .54.03 .53.02 .52.01 .51.0

0.10

0.05

0.00

X

P(X

)

S ampling Distribution of the Me an

X P(X) XP(X) X-X (X-X)2 P(X)(X-X)2

1.0 0.015625 0.015625 -3.5 12.25 0.1914061.5 0.031250 0.046875 -3.0 9.00 0.2812502.0 0.046875 0.093750 -2.5 6.25 0.2929692.5 0.062500 0.156250 -2.0 4.00 0.2500003.0 0.078125 0.234375 -1.5 2.25 0.1757813.5 0.093750 0.328125 -1.0 1.00 0.0937504.0 0.109375 0.437500 -0.5 0.25 0.0273444.5 0.125000 0.562500 0.0 0.00 0.0000005.0 0.109375 0.546875 0.5 0.25 0.0273445.5 0.093750 0.515625 1.0 1.00 0.0937506.0 0.078125 0.468750 1.5 2.25 0.1757816.5 0.062500 0.406250 2.0 4.00 0.2500007.0 0.046875 0.328125 2.5 6.25 0.2929697.5 0.031250 0.234375 3.0 9.00 0.2812508.0 0.015625 0.125000 3.5 12.25 0.191406

1.000000 4.500000 2.625000

E XV XSD X

X

X

X

( )( )

( ) .

= == =

= =

ms

s

4.52.62516202

2



• Comparing the population distribution and the sampling distribution of the mean:The sampling distribution is

more bell-shaped and symmetric.

Both have the same center.The sampling distribution of

the mean is more compact, with a smaller variance.

87654321

0.2

0.1

0.0

X

P(X

)

Uniform Distribution (1,8)

X8.07.57.06.56.05.55.04.54.03.53.02.52.01.51.0

0.10

0.05

0.00

P(X

)

Sampling Distribution of the Mean

Properties of the Sampling Distribution of the Sample Mean


The expected value of the sample mean is equal to the population mean:

E XX X

( ) The variance of the sample mean is equal to the population variance divided by

the sample size:

V XnX

X( ) 2

2

The standard deviation of the sample mean, known as the standard error of the mean, is equal to the population standard deviation divided by the square

root of the sample size:

SD XnX

X( )

Relationships between Population Parameters and the Sampling Distribution of the Sample Mean


When sampling from a normal population with mean and standard deviation , the sample mean, X, has a normal sampling

distribution:

When sampling from a normal population with mean and standard deviation , the sample mean, X, has a normal sampling

distribution:X N

n~ ( , ) 2

This means that, as the sample size increases, the

sampling distribution of the sample mean remains

centered on the population mean, but becomes more

compactly distributed around that population mean

This means that, as the sample size increases, the

sampling distribution of the sample mean remains

centered on the population mean, but becomes more

compactly distributed around that population mean

Normal population

0.4

0.3

0.2

0.1

0.0

f(X)

Sampling Distribution of the Sample Mean

Sampling Distribution: n =2



Sampling from a Normal Population

Normal population


When sampling from a population with mean and finite standard

deviation , the sampling distribution of the sample mean will

tend to a normal distribution with mean and standard deviation as

the sample size becomes large(n >30).

For “large enough” n:

When sampling from a population with mean and finite standard

deviation , the sampling distribution of the sample mean will

tend to a normal distribution with mean and standard deviation as

the sample size becomes large(n >30).

For “large enough” n:

n

)/,(~ 2 nNX

P( X)

X

0.25

0.20

0.15

0.10

0.05

0.00

n = 5

P( X)

0.2

0.1

0.0

X

n = 20

f ( X)

X-

0.4

0.3

0.2

0.1

0.0

Large n

The Central Limit Theorem


Normal Uniform Skewed

Population

n = 2

n = 30

XXXX

General

The Central Limit Theorem Applies to Sampling Distributions from Any

Population


Mercury makes a 2.4 liter V-6 engine, the Laser XRi, used in speedboats. The company’s engineers believe the engine delivers an average power of 220 horsepower and that the standard deviation of power delivered is 15 HP. A

potential buyer intends to sample 100 engines (each engine is to be run a single time). What is the probability that the sample mean will be less than 217HP?

Mercury makes a 2.4 liter V-6 engine, the Laser XRi, used in speedboats. The company’s engineers believe the engine delivers an average power of 220 horsepower and that the standard deviation of power delivered is 15 HP. A

potential buyer intends to sample 100 engines (each engine is to be run a single time). What is the probability that the sample mean will be less than 217HP?

P X PX

n n

P Z P Z

P Z

( )

( ) .

217217

217 22015

100

217 2201510

2 0 0228

The Central Limit Theorem (Example 5-1)


Example 5-2

EPS Mean Distribution

0

5

10

15

20

25

Range

Fre

qu

en

cy

2.00 - 2.49

2.50 - 2.99

3.00 - 3.49

3.50 - 3.99

4.00 - 4.49

4.50 - 4.99

5.00 - 5.49

5.50 - 5.99

6.00 - 6.49

6.50 - 6.99

7.00 - 7.49

7.50 - 7.99


If the population standard deviation, , is unknown, replace with the sample standard deviation, s. If the population is

normal, the resulting statistic:

has a t distribution with (n - 1) degrees of freedom.

If the population standard deviation, , is unknown, replace with the sample standard deviation, s. If the population is

normal, the resulting statistic:

has a t distribution with (n - 1) degrees of freedom.

• The t is a family of bell-shaped and symmetric distributions, one for each number of degree of freedom.

• The expected value of t is 0.• The variance of t is greater than 1, but

approaches 1 as the number of degrees of freedom increases. The t is flatter and has fatter tails than does the standard normal.

• The t distribution approaches a standard normal as the number of degrees of freedom increases.

• The t is a family of bell-shaped and symmetric distributions, one for each number of degree of freedom.

• The expected value of t is 0.• The variance of t is greater than 1, but

approaches 1 as the number of degrees of freedom increases. The t is flatter and has fatter tails than does the standard normal.

• The t distribution approaches a standard normal as the number of degrees of freedom increases.

nsXt

/

Standard normal

t, df=20t, df=10

Student’s t Distribution


The sample proportion is the percentage of successes in n binomial trials. It is the number of successes, X, divided by the number of trials, n.

The sample proportion is the percentage of successes in n binomial trials. It is the number of successes, X, divided by the number of trials, n.

pX

n

As the sample size, n, increases, the sampling distribution of approaches a normal distribution with mean p and standard deviation

As the sample size, n, increases, the sampling distribution of approaches a normal distribution with mean p and standard deviation

p

p pn

( )1

Sample proportion:

1514131211109876543210

0.2

0.1

0.0

P (X)

n=15, p = 0.3

X

1415

1315

1215

1115

10 15

9 15

8 15

7 15

6 15

5 15

4 15

3 15

2 15

1 15

0 15

1515 ^

p

210

0 .5

0 .4

0 .3

0 .2

0 .1

0 .0

X

P(X

)

n=2, p = 0.3

109876543210

0.3

0.2

0.1

0.0

P( X

)

n=10,p=0.3

X

The Sampling Distribution of the Sample Proportion,

p


Sample Proportion (Example 5-

3)


In recent years, convertible sports coupes have become very popular in Japan. Toyota is currently shipping Celicas to Los Angeles, where a customizer does a roof lift and ships them back to Japan. Suppose that 25% of all Japanese in a given income and

lifestyle category are interested in buying Celica convertibles. A random sample of 100 Japanese consumers in the category of interest is to be selected. What is the probability that at least 20% of those in the sample will express an interest in a Celica convertible?

In recent years, convertible sports coupes have become very popular in Japan. Toyota is currently shipping Celicas to Los Angeles, where a customizer does a roof lift and ships them back to Japan. Suppose that 25% of all Japanese in a given income and

lifestyle category are interested in buying Celica convertibles. A random sample of 100 Japanese consumers in the category of interest is to be selected. What is the probability that at least 20% of those in the sample will express an interest in a Celica convertible?

n

p

np E p

p p

nV p

p p

nSD p

=

=

= = =

-= = =

-= = =

100

0 25

100 0 25 25

1 25 75

1000 001875

10 001875 0 04330127

.

( )( . ) ($)

( ) (. )(. ). ($)

( ). . ($)

P p Pp p

p p

n

p

p p

n

P z P z

P z

($ . )$

( )

.

( )

. .

(. )(. )

.

.

( . ) .

> =-

->

-

-

= >-

= >-

= > - =

æ

è

ççç

ö

ø

÷÷÷

æ

è

ççç

ö

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0 201

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20 25

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115 0 8749

An estimator of a population parameter is a sample statistic used to estimate the parameter. The most commonly-used estimator of the:Population Parameter Sample Statistic Mean () is the Mean (X)Variance (2) is the Variance (s2)Standard Deviation () is the Standard Deviation (s)Proportion (p) is the Proportion ( )

An estimator of a population parameter is a sample statistic used to estimate the parameter. The most commonly-used estimator of the:Population Parameter Sample Statistic Mean () is the Mean (X)Variance (2) is the Variance (s2)Standard Deviation () is the Standard Deviation (s)Proportion (p) is the Proportion ( )p

• Desirable properties of estimators include:UnbiasednessEfficiencyConsistencySufficiency

• Desirable properties of estimators include:UnbiasednessEfficiencyConsistencySufficiency

5-4 Estimators and Their Properties


An estimator is said to be unbiased if its expected value is equal to the population parameter it estimates.

For example, E(X)=so the sample mean is an unbiased estimator of the population mean. Unbiasedness is an average or long-run property. The mean of any single sample will probably not equal the population mean, but the average of the means of repeated independent samples from a population will equal the population mean.

Any systematic deviation of the estimator from the population parameter of interest is called a bias.

An estimator is said to be unbiased if its expected value is equal to the population parameter it estimates.

For example, E(X)=so the sample mean is an unbiased estimator of the population mean. Unbiasedness is an average or long-run property. The mean of any single sample will probably not equal the population mean, but the average of the means of repeated independent samples from a population will equal the population mean.

Any systematic deviation of the estimator from the population parameter of interest is called a bias.

Unbiasedness


An unbiased estimator is on target on average.

A biased estimator is off target on average.

{

Bias

Unbiased and Biased Estimators


An estimator is efficient if it has a relatively small variance (and standard deviation).

An estimator is efficient if it has a relatively small variance (and standard deviation).

An efficient estimator is, on average, closer to the

parameter being estimated..

An inefficient estimator is, on average, farther from the

parameter being estimated.

Efficiency


An estimator is said to be consistent if its probability of being close to the parameter it estimates increases as the sample size increases.

An estimator is said to be consistent if its probability of being close to the parameter it estimates increases as the sample size increases.

An estimator is said to be sufficient if it contains all the information in the data about the parameter it estimates.

An estimator is said to be sufficient if it contains all the information in the data about the parameter it estimates.

n = 100n = 10

Consistency

Consistency and Sufficiency


For a normal population, both the sample mean and sample median are unbiased estimators of the population mean, but the sample mean is both more efficient (because it has a smaller variance), and sufficient. Every observation in the sample is used in the calculation of the sample mean, but only the middle value is used to find the sample median.

In general, the sample mean is the best estimator of the population mean. The sample mean is the most efficient unbiased estimator of the population mean. It is also a consistent estimator.

Properties of the Sample Mean


The sample variance (the sum of the squared deviations from the sample mean divided by (n-1) is an unbiased estimator of the

population variance. In contrast, the average squared deviation from the sample mean is a biased (though consistent) estimator of the

population variance.

The sample variance (the sum of the squared deviations from the sample mean divided by (n-1) is an unbiased estimator of the

population variance. In contrast, the average squared deviation from the sample mean is a biased (though consistent) estimator of the

population variance.

E s Ex xn

Ex xn

( )( )( )

( )

22

2

22

1=

--

æèç

öø÷=

-æèç

öø÷<

å

å

s

s

Properties of the Sample Variance


Consider a sample of size n=4 containing the following data points:

x1=10 x2=12 x3=16 x4=?

and for which the sample mean is:

Given the values of three data points and the sample mean, the value of the fourth data point can be determined:

x =x

n

12 14 164

414

12 14 164

56

x

x

x4 56 12 14 16

x4 56

xx

n

14

5-5 Degrees of Freedom

x4 = 14x4 = 14


x 14

If only two data points and the sample mean are known:

x1=10 x2=12 x3=? x4=?

The values of the remaining two data points cannot be uniquely determined:

x =x

n

12 143 4

414

12 143 4 56

x x

x x

Degrees of Freedom (Continued)


The number of degrees of freedom is equal to the total number of measurements (these are not always raw data points), less the total

number of restrictions on the measurements. A restriction is a quantity computed from the measurements.

The sample mean is a restriction on the sample measurements, so after calculating the sample mean there are only (n-1) degrees of freedom remaining with which to calculate the sample variance.

The sample variance is based on only (n-1) free data points:

The number of degrees of freedom is equal to the total number of measurements (these are not always raw data points), less the total

number of restrictions on the measurements. A restriction is a quantity computed from the measurements.

The sample mean is a restriction on the sample measurements, so after calculating the sample mean there are only (n-1) degrees of freedom remaining with which to calculate the sample variance.

The sample variance is based on only (n-1) free data points:

sx x

n2

2

1=

--

å ( )( )

Degrees of Freedom (Continued)


A sample of size 10 is given below. We are to choose three different numbers from which the deviations are to be taken. The first number is to be used for the first five sample points; the second number is to be used for the next three sample points; and the third number is to be used for the last two sample points.

A sample of size 10 is given below. We are to choose three different numbers from which the deviations are to be taken. The first number is to be used for the first five sample points; the second number is to be used for the next three sample points; and the third number is to be used for the last two sample points.

Example 5-4

Sample # 1 2 3 4 5 6 7 8 9 10

Sample Point

93 97 60 72 96 83 59 66 88 53

i. What three numbers should we choose in order to minimize the SSD (sum of squared deviations from the mean).?

• Note:

2

xxSSD


Solution: Choose the means of the corresponding sample points. These are: 83.6, 69.33, and 70.5.

ii. Calculate the SSD with chosen numbers.

Solution: SSD = 2030.367. See table on next slide for calculations.

iii. What is the df for the calculated SSD?

Solution: df = 10 – 3 = 7.

iv. Calculate an unbiased estimate of the population variance.

Solution: An unbiased estimate of the population variance is SSD/df = 2030.367/7 = 290.05.

Solution: Choose the means of the corresponding sample points. These are: 83.6, 69.33, and 70.5.

ii. Calculate the SSD with chosen numbers.

Solution: SSD = 2030.367. See table on next slide for calculations.

iii. What is the df for the calculated SSD?

Solution: df = 10 – 3 = 7.

iv. Calculate an unbiased estimate of the population variance.

Solution: An unbiased estimate of the population variance is SSD/df = 2030.367/7 = 290.05.

Example 5-4 (continued)


Example 5-4 (continued)

Sample # Sample Point Mean Deviations Deviation Squared

1 93 83.6 9.4 88.36

2 97 83.6 13.4 179.56

3 60 83.6 -23.6 556.96

4 72 83.6 -11.6 134.56

5 96 83.6 12.4 153.76

6 83 69.33 13.6667 186.7778

7 59 69.33 -10.3333 106.7778

8 66 69.33 -3.3333 11.1111

9 88 70.5 17.5 306.25

10 53 70.5 -17.5 306.25

SSD 2030.367

SSD/df 290.0524


Using the Template


Constructing a sampling distribution of the mean from a uniform population (n=10) using EXCEL (use RANDBETWEEN(0, 1) command to generate values to graph):

Histogram of Sample Means

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Sample Means (Class Midpoints)

Fre

quen

cy

CLASS MIDPOINT FREQUENCY0.15 00.2 00.25 30.3 260.35 640.4 1130.45 1830.5 2130.55 1780.6 1280.65 650.7 200.75 30.8 30.85 0

999

Using the Computer


M.Phil (Statistics)

GC University, . (Degree awarded by GC University)

M.Sc (Statistics) GC University, . (Degree awarded by GC University)

Statitical Officer(BS-17)(Economics & Marketing Division)

Livestock Production Research Institute Bahadurnagar (Okara), Livestock & Dairy Development

Department, Govt. of Punjab

Name Shakeel NoumanReligion ChristianDomicile Punjab (Lahore)Contact # 0332-4462527. 0321-9898767E.Mail [email protected] [email protected]


mailto:[email protected]

mailto:[email protected]

Sampling and sampling distributions

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